Endothelial cell density and morphometry are essential for adequate follow-up of corneal
grafts and diseases. Fast, easy-to-use and reliable instruments to measure the endothelial
cell layer are required in clinical routine. While contact instruments provide excellent
images, these have the disadvantage of directly touching the cornea. Non-contact specular
microscopes are appreciated by clinicians and patients for their convenient handling.
In this study we focussed on two non-contact instruments from Konan and Topcon, two
widely used non-contact specular microscope manufactors in this field.
The models used in this study are the Konan Noncon Robo SP8000 from the Konan Robo
SP series and the Topcon SP3000P from the Topcon SP-series, which provide high magnification
views of specular reflected light from the corneal endothelium. Both offer auto-alignment
to capture the images. Several methods of cell analysis can be applied: Manual, semi-automatic
or automatic cell counting strategies. While automatic strategies are appealing because
of ease of use, these are known to be less accurate than semi-manual or manual cell
detection 5]. Thus we used manual cell detection for both instruments in the present study.
In addition, the number of marked cells that are incorporated into the cell density
calculation can influence the results for cell density 6]–8]. It is advised to dot as many cells as possible, since the more cells are included
in the analysis the smaller the resulting variations 9]. In the present study, we therefore marked and thus included into the calculations
as many cells as possible for the two devices: All clearly visible cells on the picture
for the Konan Robo instrument were selected, and for the Topcon device, cells were
manually marked until the built-in software ceased further cell marking. These methods
resulted in a tendency for a higher number of analyzed cells for the Topcon instrument
without statistical significance.
As it is also known that inter-observer variation can occur, all examinations and
cell dotting with both instruments were performed by the same examiner.
Other study limitations are the cohort size. As the performance of measuring instruments
und thus the congruence of two machines might be dependent on cell density and regularity,
we included not only healthy, untreated corneas but a wide range of patients to include
also a wide range of endothelial cell mosaics into our comparison. The analysis covers
endothelial cell counts from as low as about 600 cells/mm
2
to around 3000 cells/mm
2
, with around one third of measurements below 2000 cells/mm
2
. According to a linear regression model to declare influencing factors on the measurement
agreement for the two instruments, neither a low ECD nor age turned out statistically
significant predictors of the difference between endothelial cell measurements of
both instruments. Nonetheless, it is possible that in special situations like e.g. certain
diseases as cornea guttata, or poor image quality (which was not included in our study)
the observed difference between the instruments might be altered. However, even with
a limited number of eyes examined, and a wide range of cell densities taken into account,
our study shows that results are rather consistent: In 94Â % of examinations, Konan
gave higher cell counts for ECD than Topcon.
Since ECD measurements are mostly used for follow-up of endothelial cell changes in
individual patients or for clinical studies, it is important to know if or which instruments
can be used interchangeably. Several studies have compared endothelial cell counts
from models of the Topcon SP series with other contact 10] or non-contact microscopes 11], 12]. Thuret et al. compared the Topcon SP2000P with a non-contact specular microscope
from Rhine-Tec, Germany. When using the semi-automatic mode for both instruments,
agreement was far better than compared to the automatic mode, but still the Rhine-Tec
showed the tendency to overestimate low and underestimate high endothelial cell densities
compared to the Topcon 12]. De Sanctis et al. compared the same Topcon instrument SP2000P with the Konan CC7000
non-contact specular microscope. In this analysis, the endothelial cell densities
measured by the Konan where statistically significant higher than those calculated
by the Topcon; depending on the examiner, the mean difference in ECD was 185 to 229
cells between the two instruments 11]. In our analysis, we used different models, but instruments from the same manufactorers,
and found similar results: We found higher ECD for our Konan model compared to the
Topcon instrument. However, we also included patients with corneal disease or following
keratoplasty to analyze a wider range of ECDs, while in the study by de Sanctis et
al. only untreated healthy subjects where enrolled. In addition, they used the semi-automatic
mode and marked 75–88 or 80 cells, respectively, while we used the manual mode and
marked a mean of 100 or 110 cells respectively per image to minimize discrepancies
caused by calculation inaccuracies due to low numbers of included cells. Apart from
the mentioned study, Konan non-contact microscopes have also been compared to contact
microscopes for ECD measurements 13], 14]. When comparing the Konan Robo SP8000 to a non-contact instrument by Zeiss regarding
mean cell area, significant differences where detected so that the authors recommend
not to use the instruments interchangeably 13]. Several studies have shown that results for ECD, hexagonality or cell polymorphism
can differ significantly. Luft et al. compared a Konan instrument (CellChek XL) to
three non-contact models from other manufacturers (Bon Optics, Tomey and Nidek) both
in healthy and compromised corneas. They found little consistency between the 4 devices
with respect to the qualitative endothelial cell parameters CV and hexagonality readings
15].
Due to the discrepancies in all parameters tested in our own study, we recommend not
to use the Konan and the Topcon interchangeably in the same patient.